The present invention relates generally to holographic storage devices and more particularly to a method and device for storing a plurality of volume holograms within a spinning, disk-shaped, photorefractive medium.
Holographic techniques for storing images are well known. Such techniques are commonly used to store images in a wide variety of different applications. Additionally, various methodologies for utilizing such holographic techniques to store digital data for use in computer systems are currently being explored.
The technique for forming holograms comprises splitting the highly coherent output beam of a laser into separate reference and object beams. The reference beam is directed onto the holographic storage medium, e.g., a photorefractive material, while the object beam is directed onto the object whose image is to be stored. Light from the object is directed to the photorefractive medium wherein an interference pattern is formed due to the interaction of the reference beam with the object beam.
When utilized in digital data storage applications, the object beam typically passes through a spatial light modulator, e.g., a liquid crystal shutter matrix, rather than being reflected off an object, in order to form the holographic image. The spatial light modulator adds the desired digital data to the object beam to facilitate storage of the digital data in the hologram formed therefrom.
Regardless of the application (i.e., the storage of images or data), subsequently directing a reference beam onto the holographic storage medium results in the reconstruction of an image representative of the originally illuminated object or stored digital data.
Also known are techniques for storing a plurality of such images within a single photorefractive medium via angle-multiplexing of the reference beam. Such angle-multiplexing is discussed in, xe2x80x9cTHEORY OF OPTICAL INFORMATION STORAGE IN SOLIDSxe2x80x9d, Applied Optics, Vol. 2, No. 4, pg. 393 (1963). The method of angle-multiplexing generally involves maintaining a constant angle for the object beam, while varying the angle of the reference beam for each sequential exposure, i.e., the formation of each separate hologram. Angle-multiplexing thus allows a large number of holograms to be stored within a common volume of photorefractive medium, thereby greatly enhancing the storage density thereof.
Also known are techniques for storing a plurality of such holograms within a spinning drum or disk shaped photorefractive medium. Examples of some holographic memories which utilize drum or disk shaped medium are provided in U.S. Pat. Nos. 3,610,722; 3,737,878; 3,848,096; 4,104,489; 4,224,480; 4,420,829; 4,449,785; 4,929,823; 5,111,445; 5,128,693; 5,285,438; 5,339,305.
However, one problem commonly associated with such contemporary disk and drum based holographic memories is that the geometry of the system is not optimized with respect to the crystalline structure of the storage medium. Further, such contemporary systems do not utilize effective path-length monitoring so as to assure the integrity of holograms within the medium and to assure reliable read-out of a plurality of different sets of angle-multiplexed holograms.
As such, although the prior art has recognized to a limited extend the problem of storing volume holograms in a spinning disk medium, the proposed solutions, to date, have been ineffective in providing a satisfactory remedy.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a volume holographic memory comprising a disk comprised of photorefractive medium and configured to spin about a central axis thereof. The spin axis is perpendicular to a central opening formed within the disk such that the disk spins in a manner similar to that of a contemporary CD-ROM.
Object beam optics are configured to direct an object beam through the outer edge of the disk and reference beam optics are similarly configured to direct a reference beam through the outer edge of the disk. The object beam and the reference beam intersect within the photorefractive medium wherein they cooperate so as to sequentially form a plurality of separate volume holograms within the spinning disk. Such volume holograms may be written to, erased from, or read from the disk while the disk is spinning, so as to provide a fast, high density memory.
An angle multiplexer varies the angle at which either the object beam or the reference beam, preferably the reference beam, is directed through the outer edge of the disk. The angle multiplexer preferably comprises a galvanometer mirror. The storage density of the photorefractive medium is substantially enhanced via the use of such angle multiplexing.
Both the object beam and the reference beam are preferably directed into the center opening of the disk after cooperating to form a hologram, and are then reflected from the center opening of the disk via a reflecting element, preferably a pair of beam splitters. During write and erase operations, both the object and reference beams may be terminated, preferably via beam blocks, after exiting the disk, since their task has been completed and they are both no longer needed. During read-out operations, one of the two beam splitters disposed within the central opening of the disk directs the reference beam from the central opening of the disk to a phase conjugator.
The phase conjugator forms a conjugate reference beam which is directed back through the beam splitter and into the photorefractive medium of the spinning disk. As the conjugate reference beam is transmitted through the photorefractive medium, a previously stored hologram formed therein causes the conjugate reference beam to be transformed into a conjugate object beam which is representative of the hologram effecting such transformation. Thus, the conjugate reference beam excites a conjugate object beam from the stored hologram. The excited conjugate object beam is then read by a sensor, preferably a two-dimensional array charge coupled device (CCD), so as to provide an electrical signal representative of the originally stored data.
As those skilled in the art will appreciate, the phase conjugator removes distortions introduced into stored holograms in a manner which facilitates the use of inexpensive, fast, (i.e., low f/#) object beam optics. Thus, the effects of distortion, such as spreading of the object beam due to undesirable diffraction, are substantially reversed by the phase conjugator.
According to the preferred embodiment of the present invention, a high-power pulsed laser is utilized for the write, erase, and read-out operations. The energy density obtained by such a high-power pulsed laser, when focused, is sufficient to cause ionization of the air in the immediately vicinity of the focus. For this reason, a pressure cell is preferably disposed at the focus of the reference beam optics, where the power density is greatest, so as to inhibit such ionization. Those skilled in the art will appreciate that air ionization is inhibited at increased pressure.
The present invention preferably comprises a liquid Stimulated Brillouin Scattering (SBS) phase conjugator, preferably comprised of methanol. Those skilled in the art will appreciate that various other phase conjugating materials are likewise suitable.
According to the preferred embodiment of the present invention, a Pockels cell is utilized to rotate the polarization of the laser beam from which the object and reference beams are formed to an orientation suitable for writing, erasing, and reading of holograms, according to well-known principles.
A spatial light modulator (SLM) is utilized for modulating, i.e., applying digital data to, the object beam. According to the preferred embodiment of the present invention, a 1024xc3x971024 pixel reflecting type spatial light modulator is utilized to facilitate the storage of 1.18 terabits of data, as discussed in detail below. Those skilled in the art will appreciate that various different resolutions of spatial light modulators are likewise suitable.
A reflecting element, preferably the spatial light modulator, is disposed upon a translation stage and configured to vary the path length of one of the object and reference beams, preferably object beam, so as to facilitate adjustment of the relative path lengths of the object and reference beams. The translation stage preferably comprises a piezoelectro translation stage to facilitate automatic measurement and precise control of relative path lengths of the object and reference beams. Those skilled in the art will appreciate that various other types of translation stages are likewise suitable.
A sensor, preferably a one-dimensional array charged coupled device (CCD), measures the intensity or diffraction efficiency of holograms formed within the disk. This facilitates the formation of holograms according to an exposure schedule wherein later formed holograms are stored at a lower intensity than earlier formed holograms. The use of an exposure schedule and the formation of multiple holograms within a single media is taught in xe2x80x9cSTORAGE OF 500 HIGH RESOLUTION HOLOGRAMS IN A LiNbO3 CRYSTALxe2x80x9d, Optics Letters, Vol. 62, No. 8, p. 105 (1991).
The use of such an exposure schedule has been found to be helpful in minimizing diffraction efficiency degradation. More particularly, according to such exposure schedules, earlier stored holograms are formed utilizing more intense object and reference beams than later stored holograms, such that each subsequent write process tends to lower the diffraction deficiency of the earlier stored holograms in a manner which substantially equalizes the diffraction deficiency of all stored holograms. Thus, according to such exposure schedules, each succeeding hologram is stored using a lower intensity than the preceding holograms.
According to the preferred embodiment of the present invention, the object beam optics and the reference beam optics are configured so as to define an interferometer. The one-dimensional CCD array detects interference fringes resulting from combining of the object and reference beams, so as to permit measurement of the relative path lengths of the object beam path and the reference beam path. Thus, the relative path lengths of the object and reference beams can be adjusted via the piezoelectric translation stage such that the object and reference beams are in a constant or desired phase relation during write operations and are 180 degrees from this constant or desired phase relationship during erase operations.
In order to maintain and/or duplicate the desired phase relationship of the object and reference beams during write processes, a plurality of plane-wave holograms are preferably formed within the spinning disk when the phase relationship of the object and reference beams is at a desired angle. The desired phase relationship can subsequently be reproduced by utilizing the plane-wave holograms as diffraction gradings, so as to define a Michaelson interferometer which is utilized to monitor the relative path lengths of the object and reference beams, as discussed in detail below. Preferably, such plane-wave holograms are formed at each radial location of the spinning disk and at the top, middle, and bottom angles for each location. However, those skilled in the art will appreciate that various different schemes for positioning and configuring such plane-wave holograms are likewise suitable.
A sensor, preferably the same one-dimensional array as used for measurement of the relative path lengths of the object beam path and reference beam path, senses the position of the reference beam after it has been transmitted through the disk, so as to provide an indication of the position of the galvanometer mirror, thereby facilitating calibration of the galvanometer mirror.
The disk is preferably comprised of LiNbO3, preferably iron-doped (LiNbO3:Fe), and is preferably approximately six centimeters in diameter and approximately two centimeters thick and preferably has a central opening of approximately two centimeters in diameter.
According to the preferred embodiment of the present invention, the disk is configured such that the reference beam and the object beam contact the upper and lower surfaces thereof during writing and erase processes, so as to facilitate the dissipation of electrical charges within the photorefractive medium generated by the photovoltaic effect. To accomplish this, grooves are preferably formed in the upper and lower surfaces of the disk so as to define an hourglass-like cross section thereof. Fillets are preferably formed within the groove so as to mitigate the formation of stress cracks.
Further, a conductive coating is preferably applied to the upper and lower surfaces of the disk and the two conductive coatings are preferably grounded and/or shorted to one another so as to facilitate the dissipation of electrical charge accumulating thereon.
Thus, according to the present invention, a device and methodology are provided for storing a plurality of volume holograms within a spinning, disk-shaped photorefractive medium in a manner which optimizes hologram storage with respect to the crystalline structure orientation of the photorefractive material. Amplitude monitoring of the stored holograms facilitates the use of an exposure schedule to minimize diffraction efficiency degradation; path length monitoring is accomplished via the use of an interferometer defined by the object beam and reference beam optics; and calibration of the scanner or galvanometer mirror utilized to direct the reference beam to the spinning disk is provided.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.